A former medical researcher's murder conviction in the cyanide poisoning death of his neurologist wife should be thrown out because he was convicted solely on circumstantial evidence, his attorney says.

Robert Ferrante, 66, was sentenced this month to life in prison after Allegheny County jurors convicted him last fall. Prosecutors said he laced Dr. Autumn Klein's energy drink with cyanide in April 2013.

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Ferrante and Klein met while Klein was a student where Ferrante worked at the VA Hospital in Bedford, Massachusetts. They were married in 2001 and moved to Pittsburgh after being recruited to join the university's neurological surgery team.

Ferrante was considered a leading researcher on ALS, or Lou Gehrig's disease, and worked at Harvard Medical School and Massachusetts General Hospital before moving to Pittsburgh with Klein.

Defense attorney Chris Eyster wrote in an appeal filed Tuesday that prosecutors presented "not one shred of evidence" that Ferrante administered poison to his wife.

"A conviction based on conjecture cannot stand," he said.

Eyster also argued that prosecutors failed to show that Klein, 41, had lethal amounts of cyanide in her blood, citing disagreement among medical specialists as to whether she died of the poison or a sudden heart dysrhythmia.

Ferrante has steadfastly denied poisoning Klein and said he ordered the cyanide for his University of Pittsburgh Medical Center laboratory so he could use it to mimic the disease's effects on healthy cells in his lab.

A spokesman for District Attorney Stephen Zappala Jr. said the argument for a new trial "is based on issues that have already been litigated."